This invention relates to a process for preparing lube base stocks.
Lubricants used in automobiles, diesel engines and other equipment are composed of base stocks and/or base oils and additives. Base stocks and base oils are typically hydrocarbons and are divided into five groups according to their sulfur content, saturates content, and viscosity index, according to the API Interchange Guidelines (API Publication 1509).
Plants that make Group I base stocks from crude oil-derived lube base stock feedstocks typically solely use solvents such as phenol or furfural to extract the lower VI components and increase the VI of the fractions to the desired specifications. Solvent extraction typically gives a product with less than 90% saturates and more than 300 ppm sulfur. The majority of the lube production is in the Group I category.
Plants that make Group II base stocks from crude oil fractions in a xe2x80x9cpre-lube base stock rangexe2x80x9d typically use hydroprocessing (hydrocracking or severe hydrotreating) to increase the VI of the fractions to the specification value. Hydroprocessing typically increases the saturate content above 90 and reduces the sulfur below 300 ppm. Combinations of solvent processing with hydroprocessing are also used to make Group II base stocks. Approximately 10% of the world lube base stock production, and about 30% of U.S. production, is in the Group II category.
Plants that make Group III base stocks typically use Hydroisomerization Dewaxing to make very high VI products. The starting feed is typically a waxy vacuum gas oil (VGO) or wax which contains essentially saturates and little sulfur. The Group III products have saturate contents above 90 and sulfur contents below 300 ppm. Fischer Tropsch wax is an ideal feed for Hydroisomerization Dewaxing to make Group III lubes. However, only a small fraction of the world""s lube supply is in the Group III category. Group IV and V plants are specialty plants, and make up even less of the world""s lube supply.
In addition to specifications on saturates, viscosity index and sulfur, lube base stocks are typically produced in a series of viscosity grades. The lowest viscosity is almost always greater than 3 cSt when measured at 40xc2x0 C., and more typically greater than 4 cSt. The highest viscosity grade is almost always less than 50 cSt when measured at 100xc2x0 C. The finished lube oil formulator takes various viscosity grade products and blends them with additives to make a finished lubricant that has a desired viscosity. The proportions of the individual base stocks and/or base oils are adjusted to achieve the desired viscosity of the finished lubricant. Since a lube base oil plant must provide base oils for a variety of customers, it is important that all viscosity grades have other properties that are approximately constant, such as viscosity index, pour point, cloud point, etc. The viscosity of the lube base stock depends on the average molecular weight of the base stock and this, in turn, depends on the boiling range.
Lube base stocks must have acceptable pour points and cloud points in addition to an acceptable viscosity index. These properties can be important for functional considerations (they impact the actual performance of the final lubricant) and can be important for general customer acceptance. Pour point is typically measured using the ASTM 97 procedure, which measures the temperature at which an oil no longer will flow when it is cooled. Cloud point is typically measured using the ASTM D 2500 procedure, which measures the temperature at which a cloud appears in the lube base stock as it is cooled.
Pour point is of obvious functional significance as the final lubricant must not become solid during storage or use. Typical lube base stocks (Groups I-III) will have pour points below +10xc2x0 F. (xe2x88x9212xc2x0 C.). These specifications are satisfactory for the majority of lube base stocks used in engine lubrication. Chemical pour point depressants can be added to lube base stocks to further reduce their pour point, but these chemical additives are expensive. For a few small volume applications intended for cold climates, lower pour points may be needed.
Cloud point is also of functional significance where an oil filter is used to remove solids from the lubricant. Lube base stocks with high cloud points may plug the oil filter. Typical lube Group I-III base stocks will have cloud points below +14xc2x0 F. (xe2x88x9210xc2x0 C.). While chemical pour point depressants are known, analogous cloud point depressants are not known. As with the pour point, these cloud point specifications are satisfactory for the majority of lube base stocks used in engine lubrication. For a few small volume applications intended for cold climates, lower cloud points may be needed.
Wax is commonly removed from lube base stocks by Solvent Dewaxing. Solvent Dewaxing to make lube base stocks has been used for over 70 years. An advantage of using Solvent Dewaxing is that the product pour and cloud points are reduced to approximately the same value. Limitations of Solvent Dewaxing include high operating costs, use of volatile and flammable solvents, environmental problems due to solvent emissions in the air and groundwater, and production of slack wax for which there is a limited market.
The traditional method of Solvent Dewaxing is being supplanted by Catalytic Dewaxing. The trend began with Conventional Hydrodewaxing and has continued recently with Hydroisomerization Dewaxing (for example, Chevron""s Isodewaxing(trademark) process). One disadvantage of Catalytic Dewaxing is the tendency for the process to generate oils that have cloud points higher than their pour points.
It would be advantageous to have processes for preparing lube base stock and lube stock compositions that minimize the limitations associated with Solvent Dewaxing, and that also provide lube base stocks with cloud points relatively close (i.e., within about 30xc2x0 C., preferably with about 20xc2x0 C., most preferably within about 10xc2x0C.) to their pour points. The smallest pour-cloud spread is preferred because this requires less dewaxing and thus permits pertaining higher lube yields which improves economics. The present invention provides such processes.
In its broadest aspect, the present invention is directed to an integrated process for producing more than one viscosity grade of lube base stock and lube stock compositions. Hydrocarbons in the lube base stock range are prepared by catalytically dewaxing feedstocks that have a 95% point below 1150xc2x0 F. and solvent dewaxing feedstocks that have a 95% point above 1150xc2x0 F. Optionally, the solvent dewaxed fraction can additionally be subjected to Hydroisomerization Dewaxing, preferably Complete Hydroisomerization Dewaxing, before or after Solvent Dewaxing. Hydrotreatment can optionally be performed on the lube base stock to remove olefins, oxygenates and other impurities. By use of different dewaxing processes depending on the 95% point, more than one viscosity grade of lube base stock can be generated while maintaining relatively consistent pour and cloud points.
In one embodiment, the process involves performing Fischer-Tropsch synthesis on syngas to provide a range of products, and isolating various fractions (i.e., fractions that have a 95% point below 1150xc2x0 F. and fractions that have a 95% point above 1150xc2x0 F.), typically via fractional distillation. The fractions can also be obtained from other sources, for example via distillation of crude oil, provided that the fractions do not include appreciable amounts (i.e., amounts which would adversely affect the Dewaxing Catalyst) of thiols or amines. The individual fractions can also include combinations of feedstocks, from Fischer-Tropsch and other sources. The resulting dewaxed hydrocarbon products can optionally be combined with an additive package to provide a lube oil composition.
Products with desired properties can be tailor made by performing the appropriate Solvent Dewaxing or Catalytic Dewaxing steps on representative samples of each fraction, blending the resulting products, and assaying them for desired properties. Once a product with optimized properties is obtained, the conditions can be scaled up to provide a desired product stream.